Multi-feed cone cassegrain antenna



Oct. 13, 1970 T. o. PAINE 3,534,375 DEPUTY ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION MULTI-FEED CONE CASSEGRAIN ANTENNA Filed July 9, 1968 2 Sheets-Sheet l FIG.I

| C T To SUBREFLE TOR ROTATING UNIT '20 F'o ll CHARLES T. STELZRIED GERALD s. LEVY MASAKAZU SMOOT KATOW IN VEN TORS BY Q 4, 9

ATTORNEYS Get. 13, 1970 110, PA|NE 3,534,375 DEPUTY ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION MULTI-FEED CONE CASSLGRAIN ANTENNA Filed July 9, 1968 2 Sheets-Sheet 2 FIG?) FOCAL LENGTH CHARLES T. STELZRIED GERALD S. LEVY MASAKAZU SMOOT KATOW I N VEN TOR-S BY QAL ATTORNEYS United States Patent O U.S. Cl. 343-779 7 Claims ABSTRACT OF THE DISCLOSURE A Cassegrain antenna with a plurality of feeds, fixedly positioned with respect to a parabolic reflector about the antennas axis of symmetry, is disclosed. The phase centers of all the feeds are in a plane, perpendicular to the axis of symmetry. A hyperbolic subreflector which is asymmetrically truncated is included. The subreflector is rotatable about the axis of symmetry so as to reflect signals between the parabolic reflector and a selected one of the feeds.

ORIGIN OF INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 stat. 435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION Field of the invention This invention generally relates to antenna design and, more particularly, to a Cassegrain antenna with a plurality of feed cones.

Description of the prior art The structure and principles of operation of a Cassegrain antenna are well known by those familiar with antenna design. All known prior art Cassegrain antennas consist of a single feed cone which is typically located at or near the vertex of a parabolic reflector. A hyperbolic subreflector, hereafter also referred to simply as the subreflector, or the hyperbola is located in front of the reflector or parabola, between its vertex and its focal point or focus.

Parallel rays coming from a target, assumed to be at infinity, are reflected by the parabola as a converging beam and are rereflected by the hyperbola converging at the position of the feed cone or feed. In essence, the hyperbolic subreflector images the feed so that it appears as a virtual image at the focal point of the parabola.

Typically, a feed cone system is designed to operate at a selected frequency in a selected frequency band. Attempts to increase the usefulness of a Cassegrain antenna, especially of the type used for space exploration, have led to the design of multi-frequency feed cones so that one cone could be used for several missions for experiments in a specified frequency band, such as the S-band.

However, from the use of such a multi-frequency feed cone, it has become clear that meaningful improved performance and the ability to perform experiments at various frequencies could only be achieved by the use of special purpose cones, each one of which is specifically designed to receive signals in a relatively narrow frequency range. However, as presently constructed, in order to change the feed system of a Cassegrain antenna from one special purpose cone to another, it is necessary to remove 'ice the entire Cassegrain feed support structure, an operation which requires a considerable amount of time.

A need therefore exists for an improved Cassegrain antenna in which feed cone changing is accomplishable in a minimum of time.

OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a new improved Cassegrain antenna.

Another object of the present invention is to provide a Cassegrain antenna with a rnulti-feed cone arrangement.

A further object of the present invention is to provide a Cassegrain antenna which is rapidly controllable to operate with any one of a plurality of feed cones.

Still a further object of the present invention is to provide a Cassegrain antenna with a relatively simple arrangement for rapidly switching the antenna to incorporate therein any one of a plurality of feed cones, supported therein.

These and other objects of the present invention are achieved by providing a Cassegrain antenna in which a plurality of feed cones, hereafter referred to generally as the multi-feed cone arrangement, are fixedly positioned with respect to the parabolic reflector. The antenna also incorporates an asymmetrically truncated hyperbolic subreflector which is rotatable about the antennas center axis to focus on one of the feed cones which is to be used. The configuration of the hyperbolic subreflector is selected so that one of its conjugate foci coincides with the parabolas focal point, while its other focus coincides with the phase center of the particular feed cone to be used. Thus, instead of changing the feed system of the antenna by replacing one special purpose cone with another, feed cone changing is accomplished in the present invention by rotating the hyperbolic subreflector from one feed cone to the other.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram which is useful in explaining a prior art Cassegrain antenna;

FIG. 2 is a diagram which is useful in explaining the improved Cassegrain antenna of the present invention; and

FIG. 3 is an illustration of an asymmetrically truncated hyperbolic subreflector actually reduced to practice in one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to explaining the novel features of the present invention, reference is made to FIG. 1 which is a simplified diagram, useful in explaining the geometry of a conventional signal feed Cassegrain antenna. Therein, numeral 11 designates the antennas parabolic reflector with a focal point P located on the antennas axis of symmetry which is designated by numeral 12. A single feed system 13 is shown located with its phase center at a point P, which together with point P represent the conjugate foci of a hyperbolic subreflector 15. The foci are shown aligned on the axis of symmetry 12.

Unlike such a prior art single feed Cassegrain antenna, in accordance with the teachings of the present invention, which may best be explained in conjunction with FIG. 2, a plurality of feeds, which are fixedly supported with respect to the parabolic reflector 11, are incorporated. In FIG. 2, only two feeds 13a and 13b are diagrammed. It

should, however, be appreciated that any number of feeds may be incorporated. In one specific embodiment which was actually reduced to practice, three feeds, spaced 120 apart, were incorporated. The phase centers of the two feeds 13a and 13b, designated in FIG. 2 by F and F are located equidistantly from the axis of symmetry 12, and in a plane which is perpendicular to the axis 12.

Unlike prior art in which the hyperbolic subreflector is fixedly positioned, in the Cassegrain antenna of the present invention a rotatable subreflector x is included. The subreflector 15x is rotatable about the axis of symmetry 12 by a subreflector rotating unit, which is designated in FIG. 2 by block 20.

The subreflector which is hyperbolic is truncated asymmetrically, a requirement dictated by the off-axis-ofsymmetry location of each of the feeds. The asymmetrically truncated hyperbolic subreflector 15x is designed so that after the subreflector is rotated and positioned so as to focus on one of the feeds, its two foci coincide with the parabolic focal point F and the phase center of the particular feed. In the arrangement diagrammed in FIG. 2 the subreflector 15x is shown focused on feed 13a, while the dashed line 15y represents the position of the subreflector, required for focusing on the second feed 13b.

From the foregoing it should thus be apparent that switching from one feed to another does not requir changing of a feed system, which as experienced in the past is very time consuming. In accordance with the present invention, switching is accomplished by rotating the asymmetrically truncated subreflector 15x so that its real focal point coincides with the phase center of the desired feed. It should further be pointed out that the rotation is performed about the antennas axis of symmetry 12. In the particular diagrammed arrangement of the two feeds, if the two are located 180 apart, all that is required to switch the antenna from one feed to the other is to rotate the subreflector 15x by 180". Likewise, 120 rotation of the subreflector 15x is required in the previously referred to embodiment in which three feeds, 120 apart, are incorporated.

It should be pointed out that the particular shape of the asymmetrically truncated hyperbolic subreflector depends on the overall antenna dimensions, including the size and dimensions of the parabolic reflector 11, which defines the location of the focal point F, and the distance between the phase center of each feed and the focal point F. It should also be pointed out that any conventional means and techniques may be employed in the implementation of unit (FIG. 2) to controllably rotate the subreflector 15x about axis 12.

For explanatory and exemplary purposes only, reference is made to FIG. 3 which is a useful illustration of the truncation of the hyperbolic subreflector 15x, incorporated in the three-feed antenna previously referred to. In FIG. 3, the letter A designates the parabolas focal point, corresponding to the previously defined point F. Although the parabolic reflector is not shown its vertex is designated by the letter H. Numeral represents one of the three feeds. The curved surface of the hyperbolic reflector is along line JCDL. Numeral designates a flange about the subreflector. The function of the flange does not form part of this invention and therefore will not be discussed in any detail. The accompanying legend provides the dimensions and angles of the specific embodiment illustrated in FIG. 3.

LEGEND Points:

AFocus of the paraboloid and the secondary focus of the hyperboloid B-Primary focus of the hyperboloid CVertex of hyperboloid DRotation axis of hyperboloid E-Geometric center of the hyperboloids area H-Vertex of the paraboloid Angles 0 500'00" COORDINATES IN REF. TO A J 107.1967 61.7663 L 122.1528 70.3839 K 00 65.7941

Inches g 253.5112 h 91.6214 i 446.3239 196.2879 k 21.8475 1 422.8580

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and consequently it is intended that the claims be interpreted to cover such modifications and equivalents.

What is claimed is:

1. In a Cassegrain antenna of the type including a parabolic reflector with a defined focal point and an axis of symmetry the improvement comprising:

a plurality of feed cones, each defining a phase center, said feed cones being fixedly positioned about said axis of symmetry so that the feed cones phase centers are equidistantly positioned from said axis of symmetry in a plane perpendicular thereto; and

rotatable reflecting means for deflecting signals between a selected one of said feed cones and said parabolic reflector.

2. The Cassegrain antenna as recited in claim 1 wherein said reflecting means include an asymmetrically truncated hyperbolic reflector defining first and second conjugate foci, said first foci coinciding with the parabolic reflector focal point, said reflecting means including means for controllably relating said hyperbolic reflector about said axis of symmetry so as to bring the second focal point of said hyperbolic reflector into coincidence with the phase center of a selected one of said feed cones.

3. A Cassegrain antenna comprising:

a main parabolic reflector having a defined focal point and a defined axis of symmetry;

a plurality of feed means through which energy is transmittable, each feed means defining a phase center, said feeds means being fixedly positioned with respect to said parabolic reflector so that their phase centers are in a plane perpendicular to said axis of symmetry;

energy reflecting means defining a first focal point and a second focal point; and

means for controllably rotating said energy reflecting means about said axis of symmetry with the first focal point of said energy reflecting means fixedly coinciding with the focal point of said parabolic reflector, and the second focal point of said energy reflecting means coinciding with the phase center of a 6 equidistantly displayed from said axis of symmetry, with all the phase centers in a common plane perpendicular to said axis of symmetry; and

rotating subreflector means including a subreflector de- 10 of said feeds.

6. The antenna as recited in claim 5 wherein said reflector is a parabolic reflector.

selected one of said feed means.

4. The Cassegrain antenna as recited in claim 3 wherein said energy reflecting means comprise an asymmetric 7. The antenna as recited in claim 6 wherein said subhyperbolic reflector rotatable about said axis of symmetry reflector is an asymmetrically truncated hyperbolic subwith the distance between its first real focal point and its 15 reflector. second virtual focal point being equal to the distance between the focal point of said parabolic reflector and the phase center of any one of said feed means whereby when References Cited UNITED STATES PATENTS said hyperbolic reflector is focused on one of said feed 3364335 1/1965 Alsberg 343 781 means its virtual and real foci coincide with the parabolic 20 reflector focal point and the phase center of said one feed FOREIGN PATENTS means, respectively. 662,023 4/ 1963 Canada.

5. A Cassegrain antenna comprising:

a main reflector for focusing signals which are directed thereto substantially in parallel at a focal point there- 25 of, said reflector defining an axis of symmetry;

a plurality of signal feeds fixedly positioned with respect to said reflector, the phase center of each feed ELI LIEBERMAN, Primary Examiner US. Cl. X.R. 

